It is well known that development of conformational structure is dependent on amino acid sequence. However, the exact relationship, which would make possible the reliable production of native conformation from a knowledge of primary structure, remains unclear. Toward this goal, we have been concerned with the role of SS bonds in the formation and maintenance of native conformational structure. Conditions have been developed for the partial and complete reduction of lysozyme, under non-denaturing conditions, such that any observed structural changes would result specifically from SS cleavage. Intermediates representing all possible stages of reduction have been isolated from partially reduced, carboxymethylated samples of lysozyme by ion exchange chromatography. The CD characteristics of these intermediates indicate a high level of cooperativity between SS bonds and secondary structure, since they resemble those of the fully reduced protein throughout all stages of reduction. The presence of intermediates during reduction, however, contrasts with literature reports for some other proteins, according to which all measured elements of structure have disappeared or reappeared simultaneously. The present findings are consistent with formation of a critical SS bond content prior to a major secondary structural transition.